The present invention relates to a method and apparatus for controlling the operation the short circulation of a paper, paperboard or the like production machine. Especially preferably the method and apparatus according to the invention are suitable for use in the approach system, i.e. so-called short circulation, of said production machines. To put it more precisely, the objective of the invention is to keep the headbox feed pressure constant.
Almost all prior art paper machine approach systems feeding pulp to a paper machine or the like, which are well described in e.g. U.S. Pat. No. 4,219,340, comprise the following components: a white water tank, a centrifugal cleaning plant with its feeding pumps and pumps between various stages, a gas separation tank with its vacuum apparatus, a headbox feed pump, a headbox screen, a paper machine headbox and white water trays. Said components are placed in connection with the paper machine and arranged to operate as follows: The fiber material used for paper making and the fillers, which are diluted with so-called white water obtained from the wire part of the paper machine, are dosed by means of a basis weight regulation valve into the white water tank usually located at the bottom level of the mill. By means of a feed pump also located at the bottom level of the mill, the fiber suspension is pumped from the white water tank to the first cleaning stage of the centrifugal cleaning plant located usually at the machine level of the mill, i.e. the location level of the paper machine, or, as in said patent, above it. Most often the centrifugal cleaning plant comprises several (most commonly 4-6) stages, each typically provided with its own feed pump. By means of pressure created by said feed pump, the fiber suspension accepted by the first cleaning stage of the centrifugal cleaning plant is further conveyed to the gas separation tank typically located at a level above the machine level. In some cases, when there is no gas separation tank, the accept from the centrifugal cleaning plant is further transported directly into the headbox or after intermediate dilution to the headbox feed pump. In the gas separation tank the fiber suspension is subjected to the effect of vacuum created by vacuum devices, most commonly liquid ring pumps, whereby both part of the gas dissolved in the suspension and the gas present in the suspension in form of small bubbles rises above the liquid level of the tank and is removed from the tank via the vacuum devices. From the gas separation tank the fiber suspension, outgassed as thoroughly as possible, flows to the headbox feed pump located at the bottom level of the mill, which feed pump pumps the fiber suspension to the headbox screen (not shown in said US patent) also located at the bottom level, wherefrom the fiber suspension flows to the machine level into the paper machine headbox.
One problem in both the above described and other prior art paper machine approach systems is that the pressure of the pulp in the headbox is prone to some fluctuations. There are many reasons for this. One reason is created by the sometimes great variations in the density of the pulp in the gas separation tank, whereby the pressure of the suction side of the headbox feed pump varies according to the density variations of the pulp. This is the case especially when the gas separation is arranged by means of a gas separation tank provided with an overflow for maintaining a constant surface level of the suspension in the tank. Another reason is created by gas separation tanks without overflow, in which the surface level is allowed to vary within certain limits. In some cases the surface level regulation system, while keeping the surface level essentially constant, does not react to the changes in the density of the pulp. Nevertheless, in both cases the inlet pressure of the headbox feed pump changes. Unless this change in the inlet pressure is taken into account, the headbox pressure changes accordingly.
Other reasons for pressure variations are e.g. swaying operation of the headbox feed pump, pulse-generating piping or apparatuses in the piping. As an example of process apparatuses, the headbox screen and the variations or swaying in the reject amount therefrom may be mentioned. Further, without gas removal, the amount of gas in the suspension creates pulsation, too.
Prior art knows attempts to regulate the headbox pressure either by means of two parallel valves positioned in the headbox feed line or a return valve connected parallel to the headbox feed pump. In both cases, power losses may turn out to be relatively great. This is the case especially when using two parallel valves, whereby both flows are being throttled. Additionally, strong throttling in the valves may cause turbulence and cavitation, which in turn may result in pressure shocks which disturb the operation of the headbox. Thus, locating the regulation valves in the mainline of the flow leading to the headbox may cause unforeseeable problems.
Such problems with valves used to bleed off pressure fluctuations of the fiber suspension have also been recognized in U.S. Pat. No. 5,753,081. It is believed that the continues adjusting of the valve when used as an active pressure attenuator results in a relatively short life span thereof. U.S. Pat. No. 5,753,081 discloses the use of a substantially pulseless pump to transport a variable volume of the fiber suspension away from the headbox feed conduit, dependant upon the sensed pressure in the headbox. The pulseless pump having a variable operating speed is connected to the fluid conduit between the feed pump and the headbox.
It is not easy to fulfill the requirement of the paper maker about the headbox pressure staying as constant as possible, especially by means of older approach system apparatuses. Problems occur also in more modern apparatuses, specifically in connection with gas separation tanks without overflow and/or when the density of the suspension varies. In the headbox pressure regulation systems of prior art, the rotational speed of the headbox feed pump is regulated in an attempt to keep the headbox pressure constant.
One example of such pressure regulation systems has been described in WO-A1-9964668 in which the dilution of the stock to the headbox consistency takes place in two stages. The first stage has an invariable flow, and in the second stage the flow is regulated by means of a control signal received from the headbox pressure regulation. After metering the component stocks and mixing them with dilution water an invariable volume is pumped, by means of the first pump in the main line, constantly to stock cleaning and to the dilution of the second stage. The dilution in the second stage is carried out at the suction side of the second feed pump connected in series with the first pump in the main line. The regulation of the pressure in the headbox controls the speed of rotation of the second feed pump in the main line. WO-A1-9964668 also includes a modification for the application of fibres in layers. The component stocks can be metered in the desired proportions in to the middle layer in the web and into the surface layers in the web if multiple separate main lines are used.
One additional problem may be considered to be arranging the relatively large-sized headbox feed pump to have adjustable rotational speed so as to react even to small changes quickly and sensitively enough. If the actual large-sized headbox feed pump were regulated in an attempt to make it respond quickly to changes in the process, the motor or transmission of the pump would not stand great loadings. Fast changes in the rotational speed of a large-sized pump, i.e. either acceleration or deceleration, subject the motor and transmission to immense loading.
Solving e.g. said problems is the objective of the method and apparatus according to the present invention, a characteristic feature of which is that at a suitable location in the paper machine approach system there is arranged a controllable parallel flow, in connection with which flow there is an actuator having an adjustable flow capacity, by means of which actuator at least one flow in the approach system is regulated so that the pressure in the headbox remains essentially constant.
Typically, the parallel flow is essentially smaller than the main flow.
Other characteristic features of the method and apparatus according to the invention are disclosed in the appended claims.